Phosphors are generally prepared by mixing the raw materials in one of two different ways: either dry mixing or wet mixing. With dry mixing, all of the phosphor raw materials are dry mixed (i.e., without liquid) in either a ball mill with pebbles or in some sort of dry mix V-blender, followed thereafter by some sort of calcining. A major disadvantage associated with dry mixing is that a completely homogeneous mixture of the materials is rarely, if at all, obtained.
With wet mixing, the raw materials are dissolved and mixed together in solution form and treated with a precipitating agent, usually an aqueous base, in order to form a precipitate which contains all of the non-oxygen elements of the phosphor. The precipitate is then dried and usually ground prior to the phosphor formation step, namely: calcining to form and crystallize the phosphor.
The calcining (or firing) procedure used with either the dry mixing or wet mixing technique in many cases involves two firings, the first of which is a Pre-firing followed by a second final firing in a reducing atmosphere in order to bring the phosphor activator into the proper valence state. The product of the first firing must be pulverized before the second firing step is performed.